Toward Energy Efficient Electrochemical Valorization of Waste Nitrate and Sulfide

With the continuously decreasing levelized cost of renewable electricity, electrocatalytic waste treatment and valorizations have attracted increasing attention as sustainable routes for converting waste molecules into valuable chemicals. Here, we report an energy-efficient strategy that couples nitrate-reduction (NO₃R) with sulfide-oxidation reaction (SOR) to simultaneously remediate pollutants and produce value-added chemicals. By utilizing a dual-catalyst composite in which cobalt polyphthalocyanine (CoPc) and copper polyphthalocyanine (CuPc) are co-anchored on carbon nanotubes, we achieve enhanced cathodic ammonia (NH₃) production that is well matched with efficient anodic thiosulfate formation. This paired NO₃R-SOR system enables the direct synthesis of ammonium thiosulfate, a valuable fertilizer feedstock, via simple mixing of the anolyte and catholyte. Kinetic analysis reveals that the improved NH₃ production originates from a relay of the ^*NO₂ from Cu- to Co-sites, allowing both active-sites to bypass their respective rate-limiting steps. Based on these insights, we demonstrate an integrated NO₃R||SOR system that substantially lowers the required cell voltage compared with conventional NO₃R||OER (oxygen evolution reaction) systems, achieving a 64% reduction in energy consumption at 200 mA cm^-2. Preliminary techno-economic analysis further suggests a substantial increase in energy-normalized product value, highlighting a sustainable approach for coupled waste treatment and chemical production.